1. Field of the Invention
This invention relates to an original reading apparatus using a solid state image pickup device, and more particularly to a reading apparatus suitable for an image forming apparatus of the type which reproduces images by accumulation of point images, such as a laser beam printer or a light-emitting diode printer.
2. Description of the Prior Art
In an original reading apparatus, the image of an original is formed on a solid stage image pickup device (such as a CCD) on which a great number of photoelectric conversion elements are densely arranged, and this image is electrically scanned on said device and converted into a time-serial electrical signal. When the image is to be reproduced by the use of this signal, the so-called dither system is often adopted to reproduce tone or gradation.
Various originals include so-called net-point originals. Because of the beats of the period of the net points of such a net-point original with the picture elements (photoelectric conversion elements) of the solid state image pickup device (or in a systematic dither method, the period of the minimum picture element in which a plurality of picture elements such as 4.times.4 or 8.times.8 picture elements are constructed in a lump), there is caused regular unevenness. That is, moire is created by the interference between the spatial net-point frequency of the image (already made into net points) and the spatial scanning line frequency of the minimum picture element constituted by the picture elements by the image pickup device. The moire frequency (the frequency of the moire so created) and the moire amplitude (the width of the moire) depend on a number of parameters, of which the size of the net points, the angle of the screen, the size of the minimum picture element and the kind of the dither are the most important. The moire appearing in the reproduced image proves a great hindrance and remarkably deteriorates the quality of the image, and therefore, numerous proposals have already been made to prevent moire during the reproduction of images. For example, a method of transforming into noise a scanning threshold value for effecting image scanning is already known. In this method, a noise signal is added to a threshold value reference signal and thus, again in this case, the image becomes unstable and moreover, the quality of the image is deteriorated.
Also, originals having harmonic tones such as photographs or net-point originals, and originals, having no harmonic tone such as characters or the like, must be read well.